Semiconductor switches have been used recently in place of relays with mechanical contacts because of their excellent reliability, durability, response speed and the like. On the other hand, a relay has the advantageous characteristics of a small on-resistance and a small parasitic capacitance (capacitance between contacts, capacitance between contacts and driving coil). For this reason, a mechanical relay is necessarily used in a measuring circuit of a tester and the like.
However, since a relay has mechanically operating contacts, there is a large possibility of trouble and operation error when compared with other components such as semiconductor switches. Such troubles and unstable operation immediately before the troubles result in a lower availability rate and reliability of a tester.
It is necessary to check the operation of a relay. The operation of a relay has been checked conventionally by connecting it as shown in FIG. 1. As shown in FIG. 1, a relay 16 is constructed of a relay coil 4 and relay contacts 18 which are electromagnetically opened and closed when the relay coil 4 is energized.
The on/off of the relay coil 4 is controlled by a relay driver power source 17 and a relay driver 21. The relay contacts 18 are connected to a contact on-off detecting power source 19 and a contact on-off detecting voltage meter 20. With this arrangement, when the relay contacts 18 become in contact with each other, a voltage across the contact on/off detecting power source 19 is applied via the relay contacts 18 to the contact on/off detecting voltage meter 20.
In checking the operation of each relay, the relay coil 4 is first energized upon operation of the relay driver 21, and then the operation of the contact on-off detecting voltage meter 20 is checked. In this manner, it becomes possible to discriminate between the normal operation and the abnormal operation of the relay 16.
It is essential to check the operation of a relay in the manner such as described above, in order to ensure the reliability of a tester which measures the performance of semiconductors, instruments, or the like. The operation of a relay already mounted on an apparatus is carried out at present in the following manners.
(1) A subject relay is removed from a tester, and it is checked in the manner as described with FIG. 1 if the relay is malfunctioning. PA1 (2) A circuit board on which a subject relay is mounted is removed from a tester. The operation of the circuit board is checked using a board checker. PA1 (3) A dedicated relay check program is made while considering the function of a d.c. measuring circuit portion of a tester. The on-off of a subject relay is checked by flowing a current through the relay contacts.
A single tester is generally equipped with several hundreds of relays. In view of this fact, the above methods (1) and (2) have a poor operation efficiency, and require a very long time. Furthermore, there is the associated problem that a tester cannot be used while it is being checked. The method (3) provides a good operation efficiency because a relay or board does not have to be removed from the tester. However, it involves a contradiction in that a tester uses the relay itself in checking the operation of the relay, resulting in a poor reliability. Furthermore, in checking the operation of a relay, a current is caused to flow through the relay contacts. Therefore, the tester is required to stop its operation, resulting in a low availability rate. Still further, a relay may enter an unstable state even before a definite malfunction. In such a case, the relay operates normally at one time and abnormally at another time. In other words, it is not infrequent that an operation error occurs between several normal operations. It can be considered in some cases that even if a relay operated normally at the time of checking, it may operate abnormally at the stage of practical use. Namely, even if a check is made, the possibility of abnormal operation cannot be excluded.